In a wireless network having radio-frequency (RF) nodes, each node typically includes a receiver and transmitter, sometimes referred to collectively as a transceiver, offering capabilities for receiving and sending digital information over radio signals. Sometimes more than one method of RF communication is supported. In an example, an RF node of such a network has both a Bluetooth radio and a WiFi radio, although these two radio types are discussed here only by way of a non-limiting example.
Depending on the particular radios used in nodes equipped with multiple transceivers, RF technologies that use different signaling methods communicate via overlapping frequency bands; and the communications in the overlapping bands interfere with each other. For example, Bluetooth signals use frequencies between 2.4000 GHz and 2.4836 GHz (the “2.4 GHz band”), while WiFi signals are broadcast using frequencies in three 22-MHz-wide sub-bands spaced out within the 2.4 GHz band. The two methods thus overlap, and concurrent Bluetooth and WiFi communications may interfere with one another.
Both Bluetooth and WiFi provide for non-interference among multiple signals of the same type. For example, multiple Bluetooth signals can coexist in the same space without confusion, and multiple WiFi signals can coexist in the same space without confusion. Each radio technology utilizes a particular coding technique, e.g. a specific form of spread spectrum unique to the particular technology, to allow receivers to disentangle received signals. However, each signal type interferes with the other. In the example, Bluetooth signals act as noise for WiFi receivers and vice versa.
If a single nodal device has multiple RF transceiver types, two or more of the RF transceivers may interfere with each other in the manner just described, causing errors (e.g., packet losses with resulting re-transmissions) or even failure. In an example, each node of a network has both a Bluetooth radio and a WiFi radio, each of which interferes with the other. In another example, some nodes in an RF network comprise Bluetooth radios while others comprise WiFi radios; or various nodes may have one or the other radio while some number of other nodes may have a combination of the two RF types, (e.g. some nodes have Bluetooth only, some have WiFi only, and other nodes have both). The ability of two or more RF types with overlapping spectra to operate in a space is termed “RF coexistence” (or simply “coexistence”) of the types. It is desirable to enable coexistence of two or more RF types with little or no interference there between.
Traditional luminaires can be turned ON and OFF, and in some cases may be dimmed, usually in response to user activation of a relatively simple input device connected into the lines supplying power to some number of luminaires. More sophisticated lighting control systems automate the operation of the luminaires throughout a building or residence based upon preset time schedules, occupancy, and/or daylight sensing. Such lighting control systems receive sensor signals at a central lighting control panel, which responds to the received signals by deciding which, if any, relays, switching devices, and/or dimming ballasts to drive in order to turn on or off and/or adjust the light levels of one or more luminaires. Such advanced controls have involved networked communications to/from the luminaires. More recent lighting systems utilizes wireless communications; and some such advanced systems utilize multiple radio transceivers in at least some nodes of the wireless network. Such a light system therefore may be subject to issues of coexistence of the radio technologies, as generally outlined above.
A variety of techniques have been proposed for providing coexistence among radio technologies operating in a single device and/or within the same space. A first such approach involved spatial isolation. Continuing with the Bluetooth plus WiFi example, if all Bluetooth devices are kept sufficiently far from all WiFi devices and/or sufficiently isolated therefrom by barriers to radio waves, there will be no significant interference between the two RF types.
Another approach used adaptive frequency hopping. In this method in the example, the Bluetooth devices measure noise in sub-bands of the 2.4 GHz band and then restrict their transmissions to sufficiently quiet portions of the 2.4 GHz band, sidestepping the interference. Where WiFi is the interference, Bluetooth will restrict its transmissions to sub-bands within the parts of the 2.4 GHz band that are not used by WiFi. Disadvantageously, this method requires more complex Bluetooth circuitry and may restrict Bluetooth signaling to a relatively small fraction of the original designated bandwidth time slot.
A further approach used Frequency isolation. Continuing the example, the WiFi transmission can be performed in a different band (i.e., 5 GHz rather than 2.4 GHz). This is more costly and less common than 2.4 GHz WiFi, and the two types of WiFi are not compatible with each other without extra gear.
A fourth approach involved time division multiplexing (TDM), which may be used to provide mutually exclusive (in the time domain) access to the overlapping frequency bands of the two radio technologies. Such access control utilizing separation or isolation in the time domain is referred to as Time-division multiple access (TDMA). In TDM coexistence, Bluetooth and WiFi take turns. TDMA is a channel access method often utilized for shared-medium networks. It allows several users to share the same frequency channel, even the exact same frequency channel by dividing each signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot.
In a first TDM method, the “two-wire” method, two wires connect the RF transceivers. Continuing with the Bluetooth plus WiFi example, the Bluetooth device places a high voltage on the first wire when it is using the 2.4 GHz band (sending or receiving), and the WiFi device refrains from operating while this line is high. This gives the Bluetooth radio exclusive access to the Bluetooth frequency band, including the overlap with the WiFi frequency band, for some period of time. The second wire similarly silences the Bluetooth device when the WiFi radio is operating. This gives the WiFi radio exclusive access to the WiFi frequency band, including the overlap with the Bluetooth frequency band, for some period of time. The radios claim use some or all of the 2.4 GHz band on a first-come, first-served basis. In a second TDM method, the “three-wire” method, three wires connect the RF transceivers. Two of the wires operate as described above for the two-wire method. The third wire enables the Bluetooth device to override the WiFi device, claiming priority.
There is room for further improvement with techniques enabling RF coexistence. There also is room for further improvement in such coexistence techniques as may be implemented in an RF nodal network of a lighting system.